1. Field of the Invention
The present invention relates to a method of initiating an implosive underwater seismic source, and also to a device for carrying out said method.
2. Description of the Prior Art
Geophysical surveys are customarily carried out using inter alia implosive sources. This term embraces any source creating a shock wave by implosion of a volume containing a vacuum, or a volume of condensable vapour, under the effect of the pressure of the liquid medium in which the source is placed.
The implosion volume may notably be formed, in known manner, by injection into the body of water of superheated steam, or superheated water under pressure, or by vaporization of seawater by means of an electric arc, or again by cavitation caused mechanically as is the case for example in methods known in the art by the names WATERGUN and FLEXICHOC.
The shock wave has the general shape indicated in FIG. 1; there are generally observed two peaks, a first peak E called the "explosion" corresponds to ejection of steam, vaporization of the mass of superheated water, vaporization of seawater by the electric arc, or the mechanical force exercised by a moving member in contact with the seawater; a second peak I, or implosion peak, follows shortly afterwards at the moment of implosion of the cavity. It is this implosion peak which corresponds to the maximum energy of the shock wave; its amplitude is generally much greater than that of the explosion peak, and it is for this reason that sources of this type are called "implosive sources".
The time interval T.sub.B separating the "firing" signal and the implosion peak is called the "bubble period"; in fact this is the period of expansion of the bubble between the moment of its generation and that of its implosion.
Theoretical calculation and experiments show that the bubble period T.sub.B varies substantially according to the law; EQU (T.sub.B)=k/P.sup.5/6 ( 1)
k being a constant independent of the pressure. This function is shown in FIG. 2, in which the bubble period is indicated (in ms) as a function of the absolute pressure P (in bars) which is also expressed as a function of the theoretical depth (in m) at which the source is immersed.
This dependency of the bubble period on the pressure can cause problems: in fact, in order to increase the amplitudes emitted, several charges are fired simultaneously and the signals are synchronised on the strongest peak, that is to say the implosion peak. For all the sources of the type described above, there is easily controlled the start of the time for the signal causing firing to operate (at present better than one millisecond) as this start is directly controlled, without a long delay, by pneumatic, mechanical or electrical means. On the other hand the occurrence of the implosion peak cannot be controlled precisely as it depends, as has just been seen, on the depth of immersion of the charges at the moment of firing, itself depending on the height and movement of surface waves and also on the movement of the towed charges (it is easy to synchronise the explosive sources for which the occurrence of the explosion peak is independent of the local pressure).
It has been found that a variation of 0.1 bar (variation in depth of 1 m) in the local pressure causes variations of 2 to 3 ms in the bubble period. This variation is significant, notably for long bubble periods, and makes the synchronization of different implosions imperfect. There results a reduction in amplitude and loss of high frequencies.
In fact, the purity of the signal emitted affects the possible precision of analysis of the seismic signals received, the signal will be more pure as the shock wave approaches a simple Dirac impulse, that is with an implosion peak as narrow as possible. Any separation, even minimal, in the occurrence of implosions from different sources alters the signal emitted from its theoretical ideal.
Up to now there have only been proposed, to improve this synchronisation, devices analysing, over a series of firings, the periods separating the real instants of occurrence of the implosion peaks from the theoretical instant and allowing correction, in advance or in retard, of initiation of a future firing by a value equal to the running average of the periods separating the previous firings. These devices, which depend on statistical analysis of the preceding firings, allow taking account of the dispersion of the parameters from one series of firings to another, notably the unequal depths of the charges. They are on the other hand not able to compensate errors due to variations of these parameters with time, from one firing to another (in the same series of firings, and not from one series of firings to another). This is the case for the depths of immersion, which vary for the same charge with time, and from one charge to another at the same instant.
It has in fact been found that the fluctuations in the bubble period which result are always much greater than errors in the moment of initiating a firing (of the order of 0.3 to 0.7 ms, due to variations in the control system at source), which may be considered negligible in comparison with the foregoing.